The present invention relates to a process for molding and vulcanizing tyres and other rubber articles in which the molded and vulcanized tyre blank is prevented from sticking to the bladder of the vulcanization press by using a cross-linked, hardened, tightly adhering, elastic release agent film bonded to the bladder by chemical interactions.
According to the prior art, tyres are molded and vulcanized by placing tyre blanks into a spraying booth in which the blanks are rotated by means of mechanical devices. A spray gun is introduced into these rotating blanks and is used for distributing the release agent solution therein. The overspray is removed by means of vacuum or a water wall. This is followed by molding and vulcanization of the blank in a vulcanization press by means of a bladder (heating membrane), which serves to heat the tyre blank, to perform vulcanization and to press the tyre under high pressure into negative shapes (compare the description of this technique in DE-OS 31 46 053).
The bladders used in the vulcanization presses are manufactured and processed by the tyre factories using their own mixtures and on the basis of their own ideas. They are formed from various rubber mixtures and have on their exterior air ducts, which are pressed or milled in, in order to remove air which could have been trapped during the cambering of the bladder. During vulcanization the bladders are exposed to high pressures and temperatures (approximately 20 bar and upto 200.degree. C.). The vulcanization times for standard car tyres are about 9 to 15 minutes. The bladders are expanded to roughly twice their size and under these loading conditions can on an average vulcanize between 300 or 600 tyres depending on the quality of the bladder and the heating medium used.
The disadvantages of this known process for molding and vulcanizing tyres and other rubber articles are especially that the uniform introduction of the release agent solution into the blanks, particularly in the vicinity of the bead area is labor-intensive and often leads to dirtying of equipment and the sides of the tyres which necessitates additional operations. This dirtying action is brought about in that the release agent solution also passes onto the gripping arms for the blank and drips during further operations and consequently dirties the blank.
The release solutions used in the aforementioned operating processes preferably contain silicones, particularly silicone oils or silicone emulsions. Both in the solvent-based release solutions and in the aqueous release solutions the silicones lead to a considerable increase in the lubricating and release properties. However, silicones have known disadvantages, which are in part concentration-dependent.
To avoid a part of the disadvantages of the known operating processes, it has been proposed in DE-OS 31 46 053 to cover the bladder with a lubricant having a certain composition and containing silicone oil. This lubricant contains about 10 to 40 parts by weight of bentonite clay having a particle size between about 100 and 500 mesh, about 15 to 45 parts by weight of polydimethylsiloxane having a viscosity of about 40,000 to 120,000 mm.sup.2 /s at 25.degree. C., about 12 to 31 parts by weight of polyethylene glycol and/or polypropylene glycol having a molecular weight between about 1500 and 2500, about 10 to 25 parts by weight of 1 or more surface active agents for the polydimethylsiloxanes and the poly(alkylene glycol) and optionally about 4 to 12 parts by weight of a stabilizing agent. The bladder is covered with this lubricant in form of an aqueous emulsion containing about 500 to 1500 parts by weight of water. This is achieved by for example spray coating and drying, for example by evaporating in air at a temperature between about 20.degree. and 110.degree. C. The bladder so coated with a lubricant layer can be used for about 6 to 9 tyre vulcanization cycles. Thereafter excess adhesion between the contacting outer surface of the bladder and the inner surface of the tyre occurs, i.e. after molding and vulcanization of the tyre, the bladder sticks to the molded and vulcanized tyre so that separation is difficult and possibly leads to damages of the bladder and/or the tyre. Therefore, after about 6 to 9 vulcanization cycles the lubricant layer has to be renewed. In the example it is stated that the aqueous emulsion applied to the bladder was dried for 1 minute at 65.degree. C. The coating already had to be renewed after four tyre vulcanization cycles. One cycle lasting for more than 1 hour.
Though the process proposed in DE-OS 31 46 053 avoids the disadvantages caused by the treatment of the tyre blanks with release agent solution, it has besides other disadvantages the big disadvantage that the lubricant cover has to be renewed already after a short time so that the operation of the vulcanization press often has to be interrupted, which of course leads to undesired production breakdowns. Further, such frequent renewal of the lubricant cover on the bladder again leads to heavy dirtying of the vulcanization press, so that indeed the dirtying problem is only shifted. Accordingly, this process has not found acceptance in practice.